The invention relates to a method for building a deck structure which is supported by a base structure which is floating or resting on the sea bottom, said deck structure being built separately and thereafter transferred to said base structure.
Such platform decks, hereinafter also called decks, have previously been constructed on the field in open sea when the supporting structure, for instance columns or the like, have been arranged on the sea bottom and extending upwardly above sea level to a suitable height for placing the platform deck. Erecting the deck in place is time consuming and difficult work since all the component parts of the deck first must be brought to the field on barges or the like and then mounted in place on top of the columns by means of floating cranes or the like and, thereafter, upon completion of erection, the deck is furnished with the necessary mechanical equipment, quarters for the crew, etc. and this equipment also has to be transported to the field by means of barges or the like and lifted in place on the deck by means of floating cranes or the like. Bad weather often forms obstacles for such operations which therefore can extend over impredictable periods of time and lead to great expense in the form of lost production and increased building costs.
Prefabricated units, so-called "pre-packets" are now more commonly used and these units contain generally connected machinery, equipment, crew quarters etc. mounted and equipped on land in order to be freighted to the field on barges during good weather conditions and lifted up on deck by means of floating cranes.
The weight of such a unit can be as high as 1400 tons or more and therefore require crane lifting capacities that are available only to a very limited extent. In addition, good weather conditions including low wind velocity and relatively calm sea conditions is an absolute prerequisite also for performance of this work.
The equipment required on a production platform is very extensive and complex. It may for instance comprise drilling equipment for the drilling of holes in different directions from the platform, extensive production machinery such as pumps and compressors, separation plants for the separation of oil and gas, other processing equipment for condensation and fractination of gas and storage facilities for products and necessary supplies. In addition, facilities for the crew, power machinery, communication equipment and the like are required.
The bulk of this equipment must be installed accurately and carefully. Thereafter it is to be made ready, controlled and test run, and this work requires a lot of time and substantial assistance from technical experts which, like the rest of the workforce, need quarters and require extra pay to work on a platform out in the ocean.
A further drawback associated with the mounting of pre-packets and other equipment on a platform deck is that the equipment is exposed to the destructive forces of weather, wind and sea. In addition, the equipment can easily be subjected to damage during the relatively difficult installation conditions usually prevailing on such a platform.
In Norwegian patent application Ser. No. 2202/71, a method is described for building a semi-submergible drilling platform where the upper platform in the traditional way is to rest on top of columns supported by floating units. The upper platform is built of two parallelepipedical bodies arranged on base blocks serving as skids during launching the same way a ship is launched from a slipway or the like. The parallelepipedical floating bodies are fixed to the upper platform and are consequently intended to follow the platform after it has been arranged on the floating body. The upper platform is thus constructed on the slipway in an inclined position so that only a few parts, such as the drilling tower, can be mounted on the deck before the platform is brought to a floating position.
The method in accordance with Norwegian patent application Ser. No. 2202/71 does require two building sites which necessitate a certain duplication of cranes and other construction equipment. The parallelepipedical floating bodies also add to the cost, i.e. because they are permanently fixed to the platform.
Another substantial drawback associated with this method is that the platform is not supported on the same points during construction as it is after installation on the floating units. Since there always will be a substantial deflection in such a large structure due to its own weight and installed equipment, the platform will change its form when it is transferred from one set of supporting points to another. Equipment having strong requirements as to accuracy of alignment while previously being installed will have to be aligned over again after the final supporting of the platform. In addition, the very strong requirements as to the stability, rigidity, and watertightness of a production platform cannot be fulfilled by a structure in accordance with Norwegian application No. 2202/71.